As is known, two fluted rollers cooperate in single face corrugators, a paper web being corrugated between the fluted rollers by a mostly plastic deformation. A press roller cooperates with the lower fluted roller, the press roller pressing the fed covering web against the corrugated web the tips of the corrugations thereof having been applied with glue beforehand. In the peripheral area of the lower fluted roller between the stamping engagement with the upper fluted roller and the press roller, care must be taken that by maintaining the shape of the corrugations, the corrugated web will smoothly abut against the corrugations of the lower fluted roller. Older single face corrugators used so-called fingers which are thin sheet metal plates which externally surround the mentioned area of the lower fluted roller in form of a half moon at axial distances. For this purpose, the upper fluted roller is comprised with annular grooves which the fingers engage. Such annular grooves of the upper fluted roller have the drawback that they lead to a corresponding deformation of the surface of the lower fluted roller. Therefore, it is necessary to adjust the fluted rollers from time to time axially towards each other in order to make the wear more even.
Modern single face corrugators are so-called fingerless machines. It is known from German patent letter 2 840 150 to arrange a vacuum chamber along the outer peripheral area of the lower fluted roller which is not covered by the corrugated cardboard web. Thereby, also a vacuum in the peripheral area of the lower fluted roller which the corrugated cardboard web engages is created through axially spaced annular grooves in the fluted roller. It is also known from German patent letter 2 713 464 to create such a vacuum in the annular grooves in that the lower fluted roller comprises an inner passage connected to the vacuum source, the passage being connected to the annular grooves by radial bores. In the peripheral area of the fluted roller not covered by the corrugated cardboard web again fingers are arranged in the annular grooves in order to prevent a "short circuit" of the vacuum. In contrast to the finger machine wherein the guiding sleets are subject to considerable wear and therefore have to be frequently replaced, the fingers define only a sealing in the machine described last which will result in a much lower wear. It is also known from German patent letter 2 951 774 to provide a plurality of longitudinal bores parallel to the axis instead of a central passage in the lower fluted roller, the longitudinal bores being connected to the annular grooves by radial bores. It is also known from European patent letter 0 009 907 to locate such axially parallel suction passages such that they intersect with the annular grooves.
In an inversion of the vacuum principle, it is known from U.S. Pat. No. letter 4 261 784 to associate a pressure chamber with the area of the lower fl ted roller which is surrounded by the corrugated cardboard web in order to press the corrugated cardboard web against the surface of the fluted roller. For producing a corresponding differential pressure, the lower fluted roller again is comprised with axially spaced annular grooves. It is also known from U.S. Pat. No. letter 4 368 094 to have nozzles engage annular grooves of the lower fluted roller, the nozzles being connected to a vacuum source.
In all mentioned cases, therefore, one of the two fluted rollers is comprised of annular grooves which have depths greater than the depths of the corrugations through which they run. Such annular grooves, however, may result in a quality deterioration of the corrugated cardboard. The corrugated cardboard web to be formed between the fluted rollers is subject to a significant tension. Therefore, the web is deformed into the annular grooves in the area thereof. In this area subsequently no gluing to the covering web will occur. Furthermore, the stability of the corrugated cardboard is weakened along the corrugations.
Now it is known from European Pat. letter No. 0 025 759 to provide short slots instead of throughgoing annular grooves, the slots being annularly arranged at the periphery of the lower fluted roller and to connect the slots alternately with vacuum or pressure passages, respectively. Also with such slots the web is being deformed into the slots by its inherent tension, however, not continuously like in the continuously extending annular grooves. On the other side, such a fluted roller comprises other disadvantages. Naturally dust and dirt will collect in the annular grooves and also in the slots after a certain time of operation whereby the function of the grooves and slots is impaired. Therefore, the fluted roller has to be cleaned from time to time. A cleaning of the slots, however, is very time-consuming. The production of a fluted roller with slots is also expensive as these will have to be separately milled piece by piece. The necessary chromium plating of such a fluted roller is also expensive as shutters have to be inserted into single slots in order to leave the slots free.
Therefore, it is an object of the invention to provide a fluted roller for a single face corrugator which reduces the impairment of the corrugated cardboard by necessary grooves to a minimum value without influencing the function of the grooves or the fluted roller, respectively.